Earth-boring tools are commonly used for forming (e.g., drilling and reaming) wellbore holes (hereinafter “wellbores”) in earth formations. Earth-boring tools include, for example, rotary drill bits, core bits, eccentric bits, bicenter bits, reamers, underreamers, and mills.
Earth-boring rotary drill bits have several configurations. One configuration is the fixed-cutter drill bit, which typically includes a plurality of wings or blades each having multiple cutting elements fixed thereon. Another configuration is the roller cone bit, which typically includes three cones mounted on supporting bit legs that extend from a bit body, which may be formed from, for example, three bit head sections that are welded together to form the bit body. Each bit leg may depend from one bit head section. Each roller cone is configured to rotate on a bearing shaft that extends from a bit leg in a radially inward and downward direction from the bit leg. The cones are typically formed from steel, but they also may be formed from a particle-matrix composite material (e.g., a cermet composite such as cemented tungsten carbide). Cutting teeth for cutting rock and other earth formations may be machined or otherwise formed in or on the outer surfaces of each cone. Alternatively, receptacles are formed in outer surfaces of each cone, and inserts formed of hard, wear-resistant material, in some instances coated with a superabrasive material such as polycrystalline diamond, are secured within the receptacles to form the cutting elements of the cones.
A rotary drill bit may be placed in a bore hole such that the cutting structures thereof are adjacent and in contact with the earth formation to be drilled. As the drill bit is rotated under longitudinal force applied to a drill string to which the rotary drill bit is secured, the cutting structures remove the adjacent formation material.
It is known in the art to apply wear-resistant materials, such as so-called “hardfacing” materials, to the formation-engaging surfaces of rotary drill bits to minimize wear of those surfaces of the drill bits caused by abrasion. For example, abrasion occurs at the formation-engaging surfaces of an earth-boring tool when those surfaces are engaged with and sliding relative to the surfaces of a subterranean formation in the presence of the solid particulate material (e.g., formation cuttings and detritus) carried by conventional drilling fluid. For example, hardfacing may be applied to cutting teeth on the cones of roller cone bits, as well as to the gage surfaces of the cones. Hardfacing also may be applied to the exterior surfaces of the curved lower end or “shirttail” of each bit leg, and other exterior surfaces of the drill bit that are likely to engage a formation surface during drilling. Hardfacing also may be applied to formation-engaging surfaces of fixed-cutter drill bits.
During drilling, drilling fluid is pumped down the wellbore through the drill string to the drill bit. The drilling fluid passes through an internal longitudinal bore within the drill bit and through other fluid conduits or passageways within the drill bit to nozzles that direct the drilling fluid out from the drill bit at relatively high velocity. The nozzles may be directed toward the cutting structures to clean debris and detritus from the cutting structures and prevent “balling” of the drill bit. The nozzles also may be directed past the cutting structures and toward the bottom of the wellbore to flush debris and detritus off from the bottom of the wellbore and up the annulus between the drill string and the casing (or exposed surfaces of the formation) within the wellbore, which may improve the mechanical efficiency of the drill bit and the rate of penetration (ROP) of the drill bit into the formation.
It is known in the art to use flow tubes to direct drilling fluid to a nozzle and out from the interior of a drill bit, particularly when it is desired to direct drilling fluid past the cones of a roller cone drill bit and toward the bottom of the wellbore. Such flow tubes may be separately formed from the bit body, and may be attached to the bit body (e.g., bit head section or bit leg) by, for example, welding the flow tubes to the bit body. A fluid course or passageway is formed through the bit body to provide fluid communication between the interior longitudinal bore of the drill bit and the fluid passageway within the flow tube.
As drilling fluid is caused to flow through the flow tubes and/or fluid passageways within a drill bit, the drilling fluid erodes away the interior surfaces of the flow tube and bit body. Such erosion may be relatively more severe at locations at which the direction of fluid flow changes, since the drilling fluid impinges on the interior surfaces of the flow tube or bit body at relatively higher angles at such locations. This erosion can eventually result in the formation of holes that extend completely through the walls of the flow tube or bit body, thereby allowing drilling fluid to exit the flow tube or bit body before passing through the nozzle, which eventually leads to failure of the designed hydraulic system of the drill bit. When the hydraulic system of the drill bit fails, the rate of penetration decreases and the drill bit becomes more susceptible to “balling.” Ultimately, the drill bit may fail and need to be replaced.